Green algae called
charophytes
and plants probably evolved from a common ancestor. This theory was
based upon the following homologies between plants and the
charophytes:

Chloroplast structure. The
pigments beta- carotene and chlorophyll b are fund in both organisms.
The DNA of both organisms match closely,and the thylakoid structures
are stacked as grana.

Biochemical similarity. The cell
wall is composed of cellulose. The charophytes cell wall contains
between 20 % and 26% cellulose. The peroxisomes of the charophytes
contain the same enzymes as that of the plants. Other algae do not
have this relationship.

Similarities in mitosis. The
nuclear membrane is completely gone by late prophase, the spindle
fibers remain until cytokinesis, and cell plate formation involves
the cooperation of microtubules, actin, microfilaments, and
vesicles.

Gamete structure. The sperm's
ultrastructure of both plant and charophytes are very
similar.

Genetic relationship. DNA and rRNA
are very similar in both organisms.

Plant as well as animal life
developed in the water. Plants must have had to gradually develop
characteristics that allowed them to survive on land. Life in shallow
water could have been part of a logical progression in the
development of adaptations for this life on land. Natural selection
may have favored shallow-water plants tolerant to periodic drying.
Some of these adaptations are: waxy cuticle, protection for
gametes,and protection for developing embryos. As the plants emerged
from the water they entered a new environment of direct sunlight,
soil rich in nutrients, and a predator free environment.

Nonvascular
Plants:

Bryophytes:
are plants that lack vascular tissue and require environmental water
to reproduce. The first three Divisions in the above chart are
nonvascular plants. General characteristics of this group include: a
waxy cuticle to slow down water loss and
gametangia
that protect developing gametes. The male gamatangium is called the
antheridium
which produces flagellated sperm. The female
archegonium
produces a single egg. Bryophytes need water to reproduce. They do
not contain any vascular tissue for up right support. These plants
usually form mats of horizontally growing tissue.

Mosses. Nonvascular plants
that form spongy mats. The plant grips the ground with elongated
cells called
rhizoids.
Photosynthesis occurs in the stem-like structures above the matted
plant bodies. They undergo an alternation of generations with the
haploid generation being the dominant form of this cycle. The
sporophytes are generally smaller and depend on the gametophyte for
water and protection.

Liverworts. Plants with
bodies divided into lobes, life cycle similar to that of the mosses
with special structures that propel the spores out of the capsule,
and the ability to reproduce asexually from structures called
gemmae.
These are small bundles of cells that can bounce out of cups on the
surface of the gametophyte when hit by rain water.

Hornworts.
These plants resemble liverworts, but the sporophytes are
horn-shaped. Their photosynthetic cells have one large chloroplast,
unlike the many smaller ones of other plants.

Early Vascular Plants:

For plants to develop and grow
above a certain height, specialized tissues had to be developed to
absorb water and nutrients from the soil and transport this material
from the ground to the upper areas of the plant. Aquatic plants used
the water, it lived in, to support its structure. Reproductive
material was transported by the water to its destination. Pollen
eliminated the need for the water to transport the gametes, while the
development of the seed allowed the vascular plants to conquer the
land. The diploid sporophyte increased in dominance relegating the
gametophyte to a microscopic structure within the sporophyte.

Earliest Vascular
Plants: The oldest
fossilized vascular plantCooksonia was
discovered in both Europe and North American Silurian rock. True
roots and leaves were absent; with the largest species being 50 cm.
tall.

Ferns. These are the most
well represented of the seedless plants. Most ferns have fronds,
compound leaves divided into several leaflets. As the frond develops
it uncoils from a structure called a fiddle head. Ferns are
homosporus with the leafy plant the sporophyte. The
sporangia
are located on the under
surface of the fronds in areas called
sori.
The gametophyte is a free-living, small, fragile structure. Water is
necessary for fertilization since the sperm must swim to the
archogonium, where fertilization takes place.

Gymnosperms: As the climate
began to dry out the large Fern forests began to be replaced by a new
type of plant the gymnosperm. This plant had vascular tissue and a
new type of reproductive structure called the seed. These plants
contained these seeds in structures called cones. The leaves of most
of the gymnosperms are needle -like structures. These needles
contained a thick cuticle and reduced stomata. The conifers are
heterosporus (male and female gametes develop from different types of
spores on different structures).

Flowering Plants: are the
most widespread and diverse. The division Anthophyta is divided into
two classes, Monocotyledonae and Dicotyledonae. The development of
the flower allow this group of plants take over most biomes on earth.
The life cycle of the Anthophyta is similar to that of the
gymnosperms. The gametophyte stage is small and microscopic. The male
gametophyte is the mature pollen grain while the female gametophyte
is the embryo sac, located in the ovule.